1. Technical Field
The invention is related to semiconductor imaging devices and in particular to a silicon imaging device which can be fabricated using a standard CMOS process.
2. Background Art
There are a number of types of semiconductor imagers, including charge coupled devices, photodiode arrays, charge injection devices and hybrid focal plane arrays. Charge coupled devices enjoy a number of advantages because they are an incumbent technology, they are capable of large formats and very small pixel size and they facilitate noiseless charge domain processing techniques (such as binning and time delay integration). However, charge coupled device imagers suffer from a number of disadvantages. For example, they exhibit destructive signal read-out and their signal fidelity decreases as the charge transfer efficiency raised to the power of the number of stages, so that they must have a nearly perfect charge transfer efficiency. They are particularly susceptible to radiation damage, they require good light shielding to avoid smear and they have high power dissipation for large arrays.
In order to ameliorate the charge transfer inefficiency problem, charge coupled device (CCD) imagers are fabricated with a specialized CCD semiconductor fabrication process to maximize their charge transfer efficiency. The difficulty is that the standard CCD process is incompatible with complementary metal oxide semiconductor (CMOS) processing, while the image signal processing electronics required for the imager are best fabricated in CMOS. Accordingly, it is impractical to integrate on-chip signal processing electronics in a CCD imager. Thus, the signal processing electronics is off-chip. Typically, each column of CCD pixels is transferred to a corresponding cell of a serial output register, whose output is amplified by a single on-chip amplifier (e.g., a source follower transistor) before being processed in off-chip signal processing electronics. As a result, the read-out frame rate is limited by the rate at which the on-chip amplifier can handle charge packets divided by the number of pixels in the imager.
The other types of imager devices have problems as well. Photodiode arrays exhibit high noise due to so-called kTC noise which makes it impossible to reset a diode or capacitor node to the same initial voltage at the beginning of each integration period. Photodiode arrays also suffer from lag. Charge injection devices also suffer from high noise, but enjoy the advantage of non-destructive readout over charge coupled devices.
Hybrid focal plane arrays exhibit less noise but are prohibitively expensive for many applications and have relatively small array sizes (e.g., 512-by-512 pixels).
What is needed is an imager device which has the low kTC noise level of a CCD without suffering from the destructive readout tendencies of a CCD.